Comets

With the Moon out of sight, the early morning of 22 April, will provide good seeing for the Lyrid meteor shower. These meteors are the result of the dusty tail of Comet Thatcher (C/1861 G1). Thatcher has an elliptical orbit, with a period of about 415 years. It will return in 2276. Space Weather has a good article about the Lyrids.

This year, the peak is expected on the evening of 21 April and the early morning of 22 April after midnight. The shower can last from 16 – 25 April. Typically, there are 10 to 20 meteors per hour. However, there is a large variation in the density of the shower. NASA has a long article (from the North American Meteor Network) concerning the shower and other events, and quotes an 1803 description from a newspaper in Richmond, Virginia on April 23rd, 1803:

Shooting stars. This electrical phenomenon was observed on Wednesday morning last at Richmond and its vicinity, in a manner that alarmed many, and astonished every person that beheld it. From one until three in the morning, those starry meteors seemed to fall from every point in the heavens, in such numbers as to resemble a shower of sky rockets…

For the techies on the audience, the radiant at maximum is at 271 degrees, i.e. RA 18h 04m, Dec +34, which is about halfway between theta and nu Hercules, and not actually in the constellation of Lyra at all.

On 15 February 2013, asteroid 2012 DA14 will pass within 22,500 kilometers of the Earth. Geostationary satellites orbit 35,800 kilometers above the surface.

This asteroid is roughly the same size as the object that burst over Tunguska Siberia in 1908.

Our friends at The Planetary Society provided the funds that allowed La Sagra Observatory in southern Spain to upgrade one of their telescopes with a new camera capable tracking fast moving objects like 2012 DA14, and determining their orbit. The new instrument has found more than ten Near-Earth Objects (NEO), along with a previously unknown comet.

The key to the new discoveries involve fast read-out Charge-Coupled Devices (CCD) and revised image filtering software used with the space-junk tracking program at La Sagra.

Before and after images of the target area for the Deep Impact Mission. The high resolution on the left shows the terrain of Tempel 1 taken by the Deep Impact spacecraft. The lower resolution image on the right, taken by StardustNExT, shows the impact crater. The outer circle annotated on the right-hand image shows the outer rim of the crater and the inner circle shows the crater floor. The crater is estimated to be 150 meters (500 feet) in diameter.

We will follow the encounter with pictures and text as they arrive. Rendezvous with comet Tempel 1 will occur at approximately 9:37 PM Phoenix time (0437 UTC Tuesday morning).

Live coverage of the Tempel 1 encounter will begin at 9:30 PM Phoenix time on NASA Television and the agency’s website.

The closest approach will be approximately 200 kilometers. In 2004, Stardust flew through the tail of comet Wild 2 and sent a capsule of material back to Earth.

The mission team expects to begin receiving images on the ground starting at around 1:00 AM Phoenix time (0800 UTC) on 15 February. A news conference previously planned for 11:00 AM Phoenix time will be held later in the day, to allow scientists more time to analyze the data and images. A new time will be announced later in the morning.

This is the story of two spacecraft, three comets and four rendezvous. So, keep your eye on the moving targets at all times.

Comet 81P Wild 2 (c#1) was visited (r#1) by the Stardust mission (s#1) in 2004 (it was launched in 1999), and sent its sample canister containing the comet bits back to Earth in 2006. The mission returned samples of the comet’s tail.

Comet 9P Tempel 1 (c#2) was impacted (r#2) on 4 July 2005 by NASA’s Deep Impact mission (s#2). Deep Impact was re-purposed for the 103P Hartley 2 (c#3) rendezvous (r#3), which took place on 4 November 2010.

Stardust NExT and Wild 2
Image Credit: NASA / JPL

Now, the Stardust spacecraft (comet Wild 2) is set to rendezvous (r#4) with comet Tempel 1 (spacecraft Deep Impact) on Valentine’s Day 14 February 2011.

And that is how you do cometary science on the cheap and with low risk.

In 2005, Tempel 1 made its closest approach to the sun. This likely changed the surface of the comet. Now, scientists will get a chance to re-image the surface of the comet with Stardust and compare the images with those taken by Deep Impact five years ago.

Following the initial images from the rendezvous of the EPOXI mission with comet Hartley 2, NASA has now released additional images of the extraordinary activity of this comet. A movie of the snow storm can be viewed here.

Below right, closeup of the snow storm swirling around comet Hartley 2. These images are from the left side of the dumbbell shaped comet, as seen from the Deep Impact spacecraft. The right side is much more active, as shown in the press release from Brown University.

The color image below shows the different sources for water vapor, dust, carbon dioxide and ice. The images are from data obtained by the High-Resolution Imager on 4 November 2010 on board Deep Impact.

Water vapor issues from a source in the middle of the dumbbell, whereas carbon dioxide (and ice and dust) comes predominately from the small end of the comet (right).

Dust is released from the active end and from the vents on the middle of the left end, while ice is spewed from various locations.

The image below compares the activity of Hartley 2 with the comet Tempel 1, which was the previous target of the Deep Impact spacecraft. This encounter occurred on 4 July 2005, and images of the encounter with the comet and the impactor can be explored on the NASA website.

Tempel 1 is 4.7 kilometers on its long axis compared to 2.2 kilometers for Hartley 2. Active jets are clearly visible on Hartley 2, where extensive image processing is required to see them on Tempel 1.

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David Brandt-Erichsen

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